1. Field of the Invention
The present invention relates to a digital camera using a focal-plane shutter.
2. Description of the Related Art
In imaging devices such as digital cameras, it is known in the art that the dynamic range of a picture image is improved by synthesizing a plurality of images obtained at different exposures as disclosed in, e.g., Japanese unexamined patent publications H11-234572 and 2005-348301 (hereinafter referred to as patent documents 1 and 2, respectively). More specifically, in patent document 1, with the use of a full-pixel readout type of CCD image pickup device from which changes of all the pixels thereof are read out at once, a first signal charge accumulated from the commencement of accumulation of electric charge to an electric-charge readout and a second signal charge accumulated from this electric-charge readout to the moment at which the mechanical shutter is shut are obtained for synthesizing an image made by the first signal charge with another image made by the second signal charge by opening a mechanical shutter to thereupon make the CCD image pickup device start accumulating electric charge and thereafter reading out the accumulated electric charge from the CCD image pickup device in the middle of the accumulating operation thereof. On the other hand, in patent document 2, with the use of a CCD image sensor that reads out changes of all the pixels in n-fields (n being an odd number), the exposure time (exposure value) for the first field of pixels and the exposure time (exposure value) for the remaining fields of pixels are made to be different from each other to synthesize an image made by the electric charge accumulated in the first field of pixels with another image made by the electric charge accumulated in the remaining fields of pixels by controlling the operations of an electric shutter and a mechanical shutter to start exposing all fields of pixels at once and thereafter shifting the accumulated charge of the first field of pixels to the vertical charge transfer channel of the CCD image sensor in the middle of the accumulating operation thereof.
The patent documents 1 and 2 are disclosed on the precondition that a lens shutter which entirely light-shields the light receiving surface of a CCD image pickup device at the same time is used as a mechanical shutter. However, in recent years, there has been a demand for an improvement in dynamic range even in an imaging device with a focal-plane shutter, in which two shutter curtains: the shutter leading curtain (first shutter curtain) and the shutter trailing curtain (second shutter curtain) travel, in that order, perform a shutter opening/closing operation. However, if an attempt is made to read out accumulated charge from the CCD image pickup device in the middle of the accumulating operation thereof, changes accumulated in all the pixels are read out from the CCD image pickup device at once, and therefore, exposure time varies according to the pixel position of the CCD image pickup device as shown in FIG. 13 (1a≠1b≠1c, 2a≠2b≠2c), so that it is impossible to obtain correct exposure.